Echo is a problem related to the perceived speech quality in telephony systems with long delays, e.g. telephony over long distances or telephony systems using long processing delays, like digital cellular systems. The echo arises in the four-to-two wire conversion in the PSTN/subscriber interface. To remove this echo, echo cancellers are usually provided in transit exchanges for long distance traffic, and in mobile services switching centers for cellular applications.
Due to the location of the echo canceller it is made adaptive; the same echo canceller is used for many different subscribers in the PSTN. This adaption is necessary not only between different calls, but also during each call, due to the non-fixed nature of the transmission network, e.g., phase slips, three-party calls, etc.
The main part of an echo canceller is an adaptive filler. The filter generates a replica of the echo, which is subtracted from the near end signal. Due to imperfect knowledge of the echo generating system, the estimated echo signal always contains errors. Hence, in practice, the echo attenuation obtained by using an adaptive filter is usually at most approximately 30 dB. For long time delays this attenuation is not enough, and in order to minimize the audible effects of these errors, a residual echo suppressor is used. The purpose of the echo suppressor is to further suppress the residual signal whenever this signal is dominated by the errors in the echo estimate. This is done by blocking the output of the echo canceller for certain levels of the output signal.
U.S. Pat. No. 4,557,072 describes an echo canceller provided with an echo suppressor in the form of an adaptive center clipper. The echo estimate produced by the echo canceller is used to control, via signal processing means, the threshold, and thereby the clipping window, of this adaptive clipper. If the power of the residual signal falls below the adaptive threshold, the residual signal is blocked or clipped, otherwise the residual signal is passed through the adaptive clipper without modification. However, the residual signal contains not only residual echo, but also background noise produced at the near end subscriber. Occasionally residual echo samples and background noise samples add constructively, and the resulting residual signal may therefore exceed the threshold. The result is undesirable sporadic transmissions of residual signals containing residual echo, which can be very annoying.
A basic problem with echo cancelling is that an echo canceller operates in a wide range of system and signal conditions:
(i) The system may have an attenuation of, say, 6-25 dB, and may be well described by a linear model. PA1 (ii) The background noise level at the near end may be between, say, -65--30 dBm0. PA1 (iii) The system may have a poor attenuation and may be poorly modelled as a linear system.
Determining proper values of thresholds that give satisfactory performance of echo suppressors in all relevant situations is a fundamental problem with control strategies based on power comparisons. Designing the threshold for case (i) would lead to imperfect suppression of the residual echo for systems described by case (iii). However, designing for case (iii) would lead to a very conservative suppressing function for systems described by case (i). Furthermore, the amount of background noise from the near end side (case (ii)) affects the performance of the adaptive filter in the echo canceller. For a high background noise level, the fluctuation of the estimated model, and not the model errors, might dominate the residual signal. Hence, even for systems described by case (i), different control strategies for the echo suppressor should be taken depending on the background noise level. From this discussion it is clear that it is difficult, if not impossible, to obtain one fixed control strategy and one set of fixed parameters that give a satisfactory performance of the echo suppressor in all relevant situations.